Optical fibers are used extensively in telecommunications systems. It is often necessary to add or drop a wavelength channel in optical links or systems. This can be achieved by a wavelength router or a Reconfigurable Optical Add-drop Multiplexer (R-OADM) that sends different wavelength channels to different locations. FIG. 6 presents a known R-OADM architecture. A composite optical signal entering R-OADM 600 from an input fiber optic line is de-multiplexed into its component channels λ1, λ2, . . . , λn by multiplexer 602a. Simultaneously, a set of channels (λ′1, λ′2, etc.) to be added are input to R-OADM 600 from add lines 604. The channels λ1 and λ′1 (if present) are directed to the 2×2 switch 606.1; the channels λ2 and λ′2 (if present) are directed to the 2×2 switch 606.2; and so on. In the example shown in FIG. 6, it is assumed that the added channels comprise only the two channels λ′1 and λ′2. Since each add operation is always paired with a concurrent drop operation, this implies that the channels λ1 and λ2 are dropped—that is, comprise the “dropped” channels. Each of the 2×2 switches 606.1–606.n can be in either one of two states-a “cross state” or a “bar” state. In the example shown in FIG. 6, since the channels λ′1, and λ′2 are added, the two switches 606.1–606.2, which receive these channels, are in their “cross” states. Since no other channels are added, the switches 606.n (and all other switches) are in their “bar” states. Thus, the channels λ1 and λ′1 and λ2 and λ′2 are switched such that the channels λ1 and λ2 are “dropped” to the drop lines 608 whilst the channels λ′1 and λ′2 are directed to the multiplexer 602b. The non-dropped or “express” channels λ3–λn are all directed to the multiplexer 602b. The multiplexer 602b multiplexes the “added” channels λ′1 and λ′2 together with the “express” channels λ3–λn so as to be output as a single composite optical signal along the output fiber optic line.
The conventional R-OADM 600 is re-configurable in the sense that the wavelengths that may be added and dropped may be chosen at the discretion of the user. Although the conventional R-OADM 600 performs its intended function adequately, it requires one 2×2 switch for each wavelength as well as two separate multiplexers. Further, the conventional R-OADM does not provide the capability of re-routing or permuting the dropped wavelengths amongst the various drop lines and does not provide the capabilities of channel power balancing or overall power control. Additional separate components must be coupled to or used in conjunction with the conventional R-OADM 600 to provide these latter capabilities. Accordingly, there is a need in the art for an improved re-configurable OADM that can perform multiple switching operations amongst the various drop ports or drop lines in a single integrated unit and that can also balance the powers of the various channels. The present invention addresses such a need.